Hydrocracking of heavy petroleum cuts is a very important refining process which makes it possible, starting from excess heavy feedstocks which are of low value, to produce lighter fractions such as gasolines, jet fuels, and light gas-oils. The refiner seeks to adapt production to demand. Compared to catalytic cracking, the advantage of catalytic hydrocracking is to provide middle distillates, jet fuels, and light gas-oils of very good quality. By contrast, the gasoline that is produced has a much lower octane number than the one that is derived from catalytic cracking.
The catalysts that are used in hydrocracking are all of the bifunctional type that combine an acid function with a hydrogenating function. The acid function is provided by substrates with large surface areas (generally 150 to 800 m.sup.2.g.sup.-1) that have a surface acidity, such as halogenated aluminas (chlorinated or fluorinated in particular), combinations of boron oxides and aluminum oxides, amorphous silica-aluminas, and zeolites. The hydrogenating function is provided either by one or more metals of group VIII of the periodic table, such as iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum, or by a combination of at least one metal from group VI of the periodic table, such as chromium, molybdenum and tungsten, and at least one metal of group VIII.
The balance between the two acid and hydrogenating functions is the main parameter that controls the activity and selectivity of the catalyst. A weak acid function and a strong hydrogenating function provide low-activity catalysts that work at a generally high temperature (greater than or equal to 390.degree. C.) and at a volumetric flow rate at low feed rate (VVH expressed by volume of feedstock to be treated per unit of volume of catalyst and per hour is generally lower than or equal to 2), but which have very good selectivity for middle distillates. Conversely, a strong acid function and a weak hydrogenating function provide very active catalysts but have poor selectivity for middle distillates. It is therefore possible, by judiciously choosing each of the functions, to adjust the activity/selectivity pair of the catalyst.
Thus, one of the great advantages of hydrocracking is to have great flexibility at various levels: flexibility at the level of the catalysts that are used, which ensures flexibility of the feedstocks that are to be treated, and at the level of the products that are obtained. An easy parameter to control is the acidity of the substrate of the catalyst.
The vast majority of the conventional catalysts for catalytic hydrocracking consist of weakly acidic substrates, such as amorphous silica-aluminas, for example. These systems are used to produce middle distillates of very good quality and, when their acidity is very low, oil bases.
The family of amorphous silica-aluminas is among the not very acid substrates. Many catalysts on the hydrocracking market consist of combined silica-alumina, either a metal of group VIII or, preferably when the contents of heteroatomic poisons of the feedstock to be treated exceed 0.5% by weight, of a combination of sulfides of the metals of groups VIB and VIII. These systems have very good selectivity for middle distillates, and the products that are formed are of good quality. The less-acid representatives of these catalysts can also produce lubricating bases. The drawback of all these catalytic systems with an amorphous substrate base is, as mentioned, their low activity.